At present, plastisols based on PVC are widely used because they offer good technical solutions to a number of problems faced by packagers, canners and bottlers. Plastisols made with PVC are flexible and tack-free, exhibit good adhesion to metal, can be formulated as a low viscosity film, sealant layer, coating or perhaps a tray, do not add a taste or smell to a packaged food or beverage, and are relatively inexpensive to produce. On the other hand, the raw materials do contain chlorine, which has become the subject of environmental concern.
Chlorine, which is essential to life and is found in common table salt, is a highly reactive element which exerts a profound effect on molecules that contain it. Many chlorinated organic compounds are highly reactive, and many, including the vinyl chloride monomer used to make the PVC polymer, are carcinogens. Further, improper incineration of materials that contain PVC can produce hydrogen chloride (a corrosive acid) and dioxins (which are poisonous). Since many communities world-wide are incinerating municipal solid waste, there is a demand to reduce the use of PVC rather than depend upon the proper incineration of waste every time.
It can be seen, therefore, that there is a need for a non-PVC plastisol, particularly where environmental concerns exist.
Plastisols are essentially blends of finely divided thermoplastic polymers suspended in a liquid medium, or plasticizer. Plastisols are particularly useful as an aid to handling polymers which would otherwise exist in a highly viscous or semi-solid state. U.S. Pat. No. 3,925,280, issued to Lundberg et al. Dec. 9, 1975 discloses that broad classes of thermoplastic materials, including graft or block copolymers as well as various ionomers, can be powdered and then blended with plasticizers to produce a suspension which is useful for making thin layers or coatings. The suspension is simply applied to a surface of an article and heated to a softening point of the polymer. A liquid-solid transition occurs upon heating, producing a plasticized semi-rigid or elastomeric product of high strength.
In U.S. Pat. No. 4,014,847, issued to Lundberg et al. Mar. 29, 1977, it is further disclosed that certain polar plasticizers such as glycerol are compatible with various ionomers including sulfonated polystyrenes. No particular end use for the resulting suspensions is disclosed.
U.S. Pat. No. 4,425,455 issued to Turner et al. Jan. 10, 1984 discloses that sulfonated thermoplastic polymers, including copolymers of sodium styrene sulfonate with styrene, vinyl toluene or t-butyl styrene, can be blended with a polar cosolvent (plasticizer) for use in a drilling mud.
The effective use of styrene/sodium styrene sulfonate (Sty/NaSS) copolymers in plastisols has been described in copending application U.S. Ser. No. 08/208,764, our Reference 8364, "Styrene/Sodium Styrene Sulfonate Copolymer for Plastisol Sealant".
Atactic polystyrene is known for use in plastisols, but, as discussed in the comparative example below, the resulting physical properties are not usable in the present application. It is difficult to plasticize atactic polystyrene with the object of producing thermo-deformation resistant tack-free films. Simple ester type plasticizers, even those which are compatible with polystyrene, do not produce an increase in flexibility. It is only after sufficient plasticizer has been added to reach a certain "threshold concentration" before flexibility is attained and at which point a highly extensible, tacky composition results. The plasticized polystyrenes have no physical strength and thermodeformation resistance.
The materials of this invention are comprised of stereoregular polystyrenes prepared by metallocene/Ziegler Natta polymerization. Combining these stereoregular polystyrenes with various amounts of plasticizers with high aromaticity, a range of desirable physical properties such as elasticity, flexibility and thermodeformation resistance are obtained.